Previous studies have shown that regulation of blood vessel tone is altered by bed rest or following space flight. Evidence suggests that cardiovascular deconditioning with bed rest results in orthostatic hypotension and that this can develop after only 4-6 hours of bed rest. The key objectives of this study are to determine the effect of short term (6 hour) head down tilt bed rest on cardiovascular function during orthostasis and to study the effect of age (20-40 and 40-60 years) and gender on these observations. We are monitoring blood pressure, ECG, Doppler (for measurement of aortic stroke volume, cerebral blood flow and femoral artery blood flow), calf circumference, respiratory rate. CO2 content and peroneal nerve muscle sympathetic tome (microneurography). The subject will be initially tilted 70 degrees head-up for 20 minutes followed by a 6 degree head down tilt for 6 hours and finally a second head-up tilt test.